1. Field of the Invention
The present invention relates to a method for forming a carbide layer on the surface of a ferrous alloy article or a cemented carbide article (by immersing the aforesaid article in a treating bath comprising molten boric acid or borate, and a carbide-forming element dissolved therein) and to a treating material for forming the treating bath.
2. Description of the Prior Art
The following surface-treating method is known. A carbon-containing ferrous alloy article or a cemented carbide article is immersed in a treating bath composed of molten boric acid or borate and a carbide-forming element, such as a Va-Group element of the Periodic Table [e.g. vanadium (V), niobium (Nb) and tantalum (Ta)], chromium or the like, dissolved therein, thus forming a carbide layer on the surface of the article. This surface-treating method can be easily and inexpensively carried out in an atmosphere. Moreover, the formed carbide layer is remarkably improved in wear resistance and seizure resistance. This surface-treating method is thus widely applicable to metal dies, jigs and the like.
In the aforesaid surface-treating method, it is advantageous to maintain the treating bath at a high temperature (without melting the article) to minimize the time required for forming a carbide layer of desired thickness. When this treating method is applied to a steel article, the temperature of the treating bath is advantageously maintained at a quenching temperature for the steel, and hardening is carried out at the same time as the surface treatment of the steel. For example, for a high-speed steel a treating bath having a temperature within a range of from 1150.degree. C. to 1300.degree. C. is employed.
However, the following problems arise when such a high-temperature treating bath is employed.
The first problem involves lowering the life of the treating bath. Namely, if the aforesaid prior art surface treating method is carried out in an atmosphere, the treating ability of the bath shows a tendency to be lowered gradually from the upper portion of the bath. While, if the bath temperature is no more than about 950.degree. C., the extent of lowering such treating ability is not so troublesome in practical use. However, when the bath temperature is 1050.degree. C. or higher, the treating ability of the bath is rapidly lowered toward the lower portion of the bath.
The second problem concerns undissolved powder of a metal, such as ferrovanadium (Fe-V) or the like, added to the treating bath as a carbide-forming element; it is deposited on the bottom of the vessel holding the bath and is sintered thereon. This sintered substance adheres intensely to the vessel and reduces the effective bath volume of the vessel. Moreover, the sintered substance also intensely adheres to the surface of the treated article and decreases the smoothness of the surface of the article. Further, the carbide layer cannot be formed on the surface of an article to which the sintered substance adheres.
The third problem is partial corrosion of the vessel and of the article. Even when a heat resistant casting alloy is used for the vessel, corrosion of the vessel is liable to occur, particularly at a portion thereof in contact with the boundary of the bath which is exposed to the atmosphere. The corrosion reaction proceeds as an exponential function of the bath temperature. If the bath temperature becomes high (about 1200.degree. C.), such corrosion becomes more pronounced. Furthermore, if the bath temperature is high, the article, a part of which is immersed in the bath, significantly corrodes at a portion thereof which is in contact with the boundary of the bath.
In addition, there is a further problem, i.e. the oxidation of the carbide layer, when the treated article is removed from the bath. It occurs at the time of removing the article from the bath. But this problem is not as serious as the aforesaid problems. Namely, the article to be treated is immersed in the treating bath to form a carbide layer on the surface thereof and, thereafter, the article is removed from the treating bath. At this time, the substance of the bath adheres to the surface of the treated carbide layer due to the viscosity of the bath. Because of the aforesaid adhesion of such a substance, the oxidation of the carbide layer, which occurs immediately after the treated article has been removed from the bath, can be prevented. The substance of the bath adhering to the surface of the carbide layer can be removed thereafter by hot water or the like. In the meantime, if the temperature of the treating bath is high, the viscosity thereof is materially decreased so that the substance of the bath only adheres to the surface of the treated article with an extremely thin layer. Therefore, the oxidation of the carbide layer is not substantially prevented.